The theoretical advantages of placing telescopes above the distorting atmosphere have been well known and practically pursued for about four or more decades. Briefly stated, these advantages include sharper images and accessibility to a broader range of wavelengths. The Hubble Space Telescope and NASA's upcoming NGST (Next Generation Space Telescope) are particularly well known examples of spaceborne telescopes. Remote sensing satellites beginning with Landsat and Spot, and more recently systems launched and operated by Space Imaging, Digital Globe, and Orbimage, represent earth-pointing examples of telescopes, known to skilled persons as “large aperture cameras.” There are, likewise, but slightly less well known, similar advantages to placing optical interferometers into space. Examples of such systems include NASA's SIM (Space Interferometry System) and SIRTF (Space Infrared Telescope Facility).
In many respects, telescopes and optical interferometers are designed with the same result in mind, namely, to measure the optical energy distribution of a spatial “scene” or of some “object.” Telescopes do so by forming a single image of an object or a scene, whereas optical interferometers explicitly measure the amplitude and phase of specific spatial frequencies of an object or a scene. Both devices can do so across a range of bands in the spectral dimension. By post-processing images derived from telescopes, one can readily obtain interferometer-like spatial frequency measurements; and by post-processing data from an optical interferometer, one can readily obtain telescope-like images, especially if a complete set of spatial frequencies has been measured.
A form of telescope implemented with non-full apertures was introduced and practically pursued before, but achieved popularity during, the 1980s. Such telescopes are referred to as “sparse array,” “phased array,” or “multi-aperture” telescopes. The basic notion of sparse array telescope design is to “coherently combine” several smaller telescopes, or sub-apertures, to achieve the resolving capabilities of a much larger telescope. An example of a multi-aperture imaging system is described in U.S. Pat. No. 5,905,591 for Multi-Aperture Imaging System. The premise underlying the operation of sparse array telescopes is that the spatial autocorrelation function of any given mirror configuration containing no drop-out points (“nulls” in spatial frequency space) achieves telescopic “imaging” or “full-coverage spatial frequency” optical interferometry in the absence of monolithic (or pseudo-monolithic, segmented) mirrors. Such a mirror configuration reduces cost and complexity. The accepted cost of implementing this relatively inexpensive approach is a reduction in light gathering capability, hence resulting in higher effective f/numbers and longer exposure times. The intended result is that much larger telescopes could be contemplated and built, thereby increasing the resolution of state of the art systems within acceptable cost budgets dictated by public security concerns and scientific endeavor priorities.
The cost virtues of sparse array telescopes have been and are now duly extolled and elucidated. At the same time, a number of various specific designs that attend to the unique design challenges presented by very large, space-based structures have been presented and sometimes implemented, at least in simulations. Noteworthy among these challenges is the need to position many optical mirrors to accuracies initially approaching and usually much finer than the wavelengths of visible light. This challenge has been referred to as “phasing” or what most people would call “maintaining focus.” Moreover, the long-established optical interferometric principle of pointing only the sub-apertures (i.e., not the whole structure) and allowing delay lines to maintain coherence is a clear design requirement for most, if not all, realistic approaches to 10-meter and larger outside-aperture class systems. In addition to the generic and given requirements for a sparse array telescope, various provisions have been envisioned, built, and tested in structures that are to be initially compactly stowed in a given structure for launch and later deployed into an operational configuration.
All of the foregoing basic requirements were well described in the 1980s, and a wide variety of specific design implementations approaching these requirements have ensued. With only a few exceptions, which tend to be classic optical interferometers in character, the sheer cost and complexity of actually building, testing, launching, and operating sparse array telescopes have, to date, permitted production of no known operational system. It has generally been found that actual structural implementation of these conceptual designs is far more difficult than simply describing the now well-understood theoretical requirements that the work of the 1980s and 1990s outlined.